Straight-line behavior

If you had continued to reduce the temperature of your real gas in this experiment (air mixture of mostly nitrogen and oxygen) to lower and lower temperatures, you would observe sudden drops in pressure at 90 K and 77 K. In other words, the behavior of the real gas would deviate significantly from the predicted straight-line behavior of the ideal gas equation determined in Question 3 and your extrapolated graph. Why ? [Hint: Consider what happens to water vapor(H2O(g)) when it is cooled to 0.0 C.]